1. Field of the Invention
The invention broadly relates to a system for guiding boats through a harbor, channel, or dangerous waters especially during periods of low visibility such as during fog or inclement weather or during the nighttime when shore markers or buoys cannot be sighted visually. More particularly, the invention relates to a radio beacon navigation system in which a vessel may determine its position from a determination of the direction at which radio signals are being transmitted in which the position of the source of radio signals has been previously charted.
2. Description of the Prior Art
Providing safe navigation for ships during periods of low visibility has been a difficult problem for many centuries. Thousands of years ago lighthouses were invented as a means for providing a visual indication to ships of a number of possible conditions. Lighthouses could either mark the entrance to a harbor or warn of many types of danger such as hidden rocks or shoals. Important as lighthouses have been in the past and still are at the present they are incapable of providing a vessel with a precise indication of its position. Furthermore, the usefulness of a lighthouse is severely limited during periods of dense fog despite the use of extremely high powered lamps which are not available. Furthermore, such lights are of little use in guiding a boat through a narrow winding channel in which the direction of the boat must be changed continually.
Many types of buoys have been in use for hundreds of years in guiding boats through harbors and other channels. A buoy in its earliest and simplest form was a floating marker anchored to the bottom of the harbor or channel. Various shapes were used depending upon what the presence of the buoy was an indication of. Various color combinations were also used. Although somewhat precise navigation was possible during periods of high visibility, such buoys were completely useless when they were hidden by darkness or obscured weather conditions.
In later days bells were attached to the buoys to give an audio indication of the presence of the buoy even when the buoy could not be seen. However, accurate navigation was not possible since it is difficult to determine the location from which such a sound is emanating. Moreover, if a large number of such buoys were employed in one harbor or channel it would be difficult to identify individual buoys as unique and identifiable audio signals are difficult to achieve using bells.
At a still later date various types of lights were attached to buoys. The lights could be flashing or of different colors. Although such lights were quite useful they consumed large amounts of power requiring constant maintenance with attendant high expense.
In modern days radio navigation systems have been developed for guiding ships both upon the high seas and through harbors and channels. In the most well known of these systems, the LORAN system, shore stations transmit identifying pulses in time sequence. The position of the ship may be computed from the difference in arrival times of pulses received from the various shore stations. This system is used primarily for determining the gross position of a vessel such as is needed for navigation upon the high seas. Also, the system requires relatively expensive receiving equipment made so by the requirement for extremely precise measurements upon the relative arrival times of the various received pulses.
Other radio direction finding systems have been developed particularly adapted for use in harbor and channel navigation. In one type of such system a plurality of shore stations continually transmit signals in phase synchronization with one another. The vessel's position is determined by comparing the phases of the received signal and determining the direction to each shore station using radio direction finding techniques. The receiving equipment required for such systems is also expensive because of the inherent difficulty in making an accurate phase measurement from two or more signals arriving within a short time period.
A second category of the system which has been developed for harbor and shore navigation employs one or more shore transmitting stations which transmit a pulse at a predetermined time. Highly accurate receiving means carried aboard the vessel measure the transit time between the shore station and ship and from that measurement determines the distance between shore station and ship. Unfortunately, this system is inordinatey expensive in that it requires the use of atomic clocks within the shipboard receiver. All of these systems are much too expensive for use aboard small vessels and pleasue craft.